1. Field of the Invention
The present invention generally relates to an ATM switch, and, more particularly, to technologies in which a plurality of ATM switches are interconnected to realize a large-scale ATM switch.
2. Description of the Related Art
It is common to configure a large-scale ATM switch by interconnecting basic switches. FIG. 28 is a block diagram of an ATM switch in which basic switches are interconnected.
Conventionally, as shown in FIG. 28, output ports of basic switches #11-#1N at a front stage are connected to input ports of basic switches #21-#2N at a back stage in a mesh manner. In other words, the number of the output ports of the basic switches #11-#1N is the same as the number of the input ports of the basic switches #21-#2N, such that the output ports are in a one-to-one correspondence with the input ports, and each of the basic switches #11-#1N selects an output port for a cell which is entered into one of the basic switches #11-#1N so that a basic switch among the basic switches #21-#2N is decided for the cell to be sent out to.
For example, in the case of using basic switches of 8 inputs by 8 outputs (8xc3x978), an ATM switch of 64 inputs by 64 outputs as a whole can be realized by interconnecting 8 basic switches #11-#18 in the first stage and 8 basic switches #21-#28 in the back stage.
Regarding the switch interconnection, a technology using a barrel shifter and a wavelength-multiplexing-optical-fiber is illustrated in N. Yamanaka, S. Yasukawa, E. Oki and T. Kawamura, xe2x80x9cOPTIMA:Tb/s ATM Switching System Architecture: Based on Highly Statistical Optical WDM Interconnection,xe2x80x9d Proc. IEEE ISS""97, System Architecture, 1997 and S. Yasukawa, N. Yamanaka, xe2x80x9c640 Gb/s Ultra-High-Speed Optical interconnection System using Wide-Channel-Spacing Wavelength Division Multiplexing,xe2x80x9d Proc.IEEE BSS""97, p.p.101-105, December 1997.
FIG. 29 is a block diagram showing an ATM switch in which basic switches are interconnected in a multistage manner. In the ATM switch, the basic switches are interconnected via WDM (wavelength division multiplexing) links by using a barrel shifter which switches signals in the WDM links according to wavelength, and, in the basic switches of the front stage, destinations of signals are determined by selecting wavelengths by which the signals are sent. Each basic switch in the back stage is connected to every basic switch in the front stage by logical links of different wavelengths. According to the above-mentioned configuration, a number of physical links between the basic switches can be eliminated.
In the following, the barrel shifter will be described with reference to FIG. 30. FIG. 30 shows how optical signals are switched within the barrel shifter. FIG. 30 shows an example in which the barrel shifter has 2 input lines I0, I1 and 4 output lines O0-O3. Here, optical signals of wavelengths xcex0-xcex3 are transmitted on each of the input lines I0, I1, and optical signals of wavelengths xcex0, xcex1, xcex2, xcex3 on the input lines I0 are respectively sent to the output lines O0, O1, O2, O3. Further, optical signals of wavelengths xcex0, xcex1, xcex2, xcex3 on the input line I1 are respectively sent to the output lines O1, O2, O3, O0. Therefore, for example, a wavelength which goes to the output line O1 is xcex1 among the wavelengths xcex0, xcex1, xcex2, xcex3 which are transmitted on the input line I0, and a wavelength which goes to the output line O1 is xcex0 among the wavelengths xcex0, xcex1, xcex2, xcex3 which are transmitted on the input line I1.
The barrel shifter is known to those skilled in the art, so only a brief description on the barrel shifter will be given in the following. On the description below, H. Takahashi, et al., xe2x80x9cWavelength MUX and DMUX by using Arrayed Waveguide Grating,xe2x80x9d IEICE, PST-91-48, pp.41-46 can be refereed to.
The barrel shifter is one of optical devices called xe2x80x9cArrayed Waveguide Grating (AWG).xe2x80x9d The conceptual diagram of the AWG is shown in FIG. 31. Generally, the AWG is made as a wavelength multiplexer-and-demultiplexer, and is an integrated on-board circuit which includes input/output waveguides and slab waveguides which act as collimators/light-gathering lenses.
As shown in FIG. 31, the AWG includes a plurality of different-length waveguides which are arranged at regular intervals. Similar to a diffraction grating, phase differences between the waveguides cause dispersion. Therefore, a wavelength-multiplexed signal from the input waveguide is demultiplexed, and the demultiplexed signals are extracted from different output waveguides. If the AWG is used in the reverse direction, the AWG acts as a wavelength multiplexer. Because the slab waveguide has the shape of a sector which has a center of curvature at the endpoint of the waveguide and the axis of the waveguide points to the center of curvature, the slab waveguide has a light-gathering function in a manner similar to a concave mirror. Generally, taper waveguides are inserted between the waveguides and the slab waveguide in order to decrease connection losses.
A wavelength interval xcex94xcex, which is one of the most important parameters in the wavelength multiplexer-and-demultiplexer by using the AWG, is represented as follows:
xcex94xcex=xcex94X/(fxc2x7m/nxxc2x7d)xe2x80x83xe2x80x83(1)
m=(ncxc2x7xcex94L)/xcex0xe2x80x83xe2x80x83(2)
wherein d is a pitch of the grating of the AWG, xcex94L is a difference of length between the waveguides, f is a focal length (a radius of curvature) of the slab waveguides, xcex94X is an interval between the waveguides, nx is an effective refractive index of the slab waveguide, the denominator of the right side of the equation (1), (fxc2x7m/nxxc2x7d), is a linear dispersion which is a proportionally constant of a relationship between the wavelength and the light-gathering position, nc is an effective refractive index of the waveguide, xcex0 is a central wavelength of the AWG and can be obtained from the central output waveguide, and m is a diffraction order indicating the number of wavelengths which represents the amount of the phase difference between the neighboring waveguides. The larger m is, the larger the linear dispersion is. Therefore, signals of many wavelengths the intervals of which are small can be multiplexed-and-demultiplexed as m increases. In other words, the larger m is, the higher the wavelength resolution of the AWG is. Regarding an ordinary diffraction grating, it is necessary to decrease the size of the pitch in order to increase the resolution. Therefore, the resolution is limited by the technology of the pitch making. But, regarding the AWG, a high resolution can be easily realized by increasing the diffraction order by increasing the length of the waveguide. This is the main difference between the AWG and the ordinary diffraction grating.
As shown in the equation (2), a plurality of central wavelengths may exist in the AWG because the m can take any number. For example, in the case of a design in which xcex94L=126 xcexcm and nc=1.45, if m=118, xcex0=1548.3 nm, and if m=119, xcex0=1535.3 nm, and a plurality of optical signals which include 1548.3-nm and 1535.3-nm signals are output from the central output port. Here, the bandwidth which can be used without overlapping is 13 nm, and, in the case of a 0.8-nm wavelength-interval WDM (wavelength division multiplexing) technology, the biggest number of wavelengths is xe2x80x9c16xe2x80x9d. As mentioned above, the larger m is, the higher the resolution is, but the narrower the bandwidth which can be used without overlapping is. Therefore, the m needs to be set carefully.
The barrel shifter used here is the AWG which utilizes a characteristic that same wavelength signals per bandwidth which is usable without overlapping are cyclically output as shown in FIG. 32.
FIG. 33 shows an 8-input by 8-output barrel shifter. The barrel shifter can be used for interconnecting the above-mentioned front stage and the back stage, each of which stages includes 8 basic 8xc3x978 switches.
It is necessary to increase the number of ports in order to enlarge further a multistage ATM switch. However, according to the above-mentioned conventional method, the number of the ports is limited by technical limitations and cost increase.
Further, there is a limitation on the number of multiplexed wavelengths when using the barrel shifter. According to the conventional method, only up to N2 ports can be interconnected within an ATM switch by using an N-wavelength-multiplexing technique.
It is an object of the present invention to provide an ATM switch which is able to expand the scale of the switch without increasing ports of a basic switch and wavelengths for multiplexing, and an ATM switch in which the hardware can be realized at a low cost. It is another object of the present invention to provide an ATM switch in which the size of buffers can be reduced, and a plurality of ATM connections of different ATM service classes are efficiently accommodated. Further, it is an object of the present invention to provide an ATM switch in which congestion of ATM connections rarely occurs. The above objects of the present invention are achieved by an ATM switch including S stages (S is an integer and 2 less than =S), wherein each stage includes a plurality of basic switches, the ATM switch including:
Mxc3x97N (each of M and N is an integer) basic switches per each stage; and
an interconnecting part for interconnecting between the stages,
wherein the Mxc3x97N basic switches have N groups, and the interconnecting part connects a jth (j is an integer and 1 less than =j less than =N) output port of each basic switch belonging to an ith (i is an integer and 1 less than =i less than =N) group at an (sxe2x88x921)th (s is an integer and 2 less than =s less than =S) stage to at least one ith input port of the basic switches belonging to a jth group at an sth stage.
The above-mentioned interconnecting part further includes a part, which is provided at each input port, for passing cells that are destined for a desired destination selectively among other cells.
And, the above-mentioned interconnecting part further includes a part, which is provided at each output port, for passing cells that are destined for a desired destination selectively among other cells.
According to the above-mentioned invention, when there are M basic switches in a group, an output port of a basic switch at the (sxe2x88x921)th stage is connected to an input port of each of M basic switches belonging to a group of basic switches at the sth stage. Therefore, basic switches each of which basic switches has N (the number of groups of a stage) output ports, can be used for the ATM switch. In this way, the ATM switch which has MN2 inputs by MN2 outputs can be realized while each basic switch having N outputs by N outputs. Because an output port of a basic switch at the (sxe2x88x921)th stage is connected to M input ports, each of which input ports belongs to each of M basic switches in a group at the sth stage, there is a connection relationship of 1:M between the output port and the input ports. In other words, a cell output from the output port is distributed to the M input ports. Therefore, a part for determining whether the distributed cell arriving at a basic switch is destined for the basic switch or not may be provided. The part may receive the cell if the cell is destined for the basic switch and discard the cell if not.
The above-mentioned interconnecting part further includes a part for converting an electronic-signal cell which is output from the output port into a first wavelength-multiplexed optical signal which includes a plurality of optical signals which have different wavelengths and outputting the first wavelength-multiplexed optical signal;
a part for generating a second wavelength-multiplexed optical signal by switching optical signals and outputting the second wavelength-multiplexed optical signal; and
a part for converting said second wavelength-multiplexed optical signal into electronic-signal cells and inputting at least one of said electronic-signal cells to said input port.
Accordingly, by connecting basic switches optically, the ATM switch of the invention can be realized easily and at a low cost.
The above-mentioned ATM switch may have a part for converting each electronic-signal cell output from the output port into an optical-signal cell which has a wavelength corresponding to the destination of the electronic-signal cell, so that a basic switch of the receiving side does not need to determine whether the arriving cell is destined for the basic switch.
The above-mentioned ATM switch further includes a speed-conversion part for converting the speed of electronic-signal cells output from the output port. According to the invention, by decreasing the cell transmitting rate, the ATM switch can be realized at a low cost because expensive hardware for high-speed cell transmission is not necessary.
The speed-conversion part may include at least a cell buffer, and a part for controlling the ratio between the writing clock speed of the cell buffer and a reading clock speed of the cell buffer.
Also, the speed-conversion part may include at least a cell buffer, and a part for controlling the ratio between the number of bit expansion data at the input side of the cell buffer and the number of bit expansion data at the output side of the cell buffer.
By decreasing the number of the bit expansion data, the cell transmitting rate can be decreased.
Further, the ATM switch may include a monitoring part for monitoring the arriving rate of the electronic-signal cells output from the output port, and a part for converting the speed of the electronic-signal cells according to the arriving rate.
According to the invention, a burst of cells can be distributed to cells at an average cell rate. Therefore, the capacity of a cell buffer downstream from the cell buffer having the monitoring part can be decreased by avoiding receiving the burst of cells.
The ATM switch may further includes a detecting part for detecting traffic congestion of a route between the basic switches by using the monitored cell rate, and a part for prohibiting the ATM switch from establishing an ATM connection on the route in which the traffic congestion is detected.
According to the invention, discarded cells can be decreased within the ATM switch because an ATM connection is not established on a congested route. The monitoring part and the detecting part may be provided at the input port of the basic switch.
The ATM switch may further include:
a monitoring part for monitoring the residual bandwidth of a port of the basic switch;
a part for sending an RM (Resource Management) cell to a plurality of routes;
a sending-back part for receiving and sending back the RM cell; and
a prohibiting part for prohibiting the ATM switch from establishing an ATM connection on a specific route according to the RM cell which is sent back,
wherein, if the monitoring part detects that the residual bandwidth is less than a predetermined value, the sending-back part includes information of traffic congestion into the RM cell and sends back the RM cell, and the prohibiting part prohibits the ATM switch from establishing an ATM connection on a route indicated by the content of the RM cell which is sent back.
According to the invention, information of congestion can be transmitted within the ATM switch without using a link specifically for the information. Further, according to the invention, the ATM connections can be controlled within the whole ATM switch, and the destination of the information of congestion can be limited to the input port. Therefore, the transmission can be performed easily.
According to the above-mentioned invention, the ATM switch of MN2 inputs by MN2 outputs can be realized easily and at a low cost without increasing the number of ports of the basic switch and the number of the wavelengths. Further, by decreasing the cell transmission rate using the speed-conversion part, by converting incoming cells at unbalanced intervals into cells at balanced intervals, and by establishing routes according to ATM service classes, and so on, the hardware configuration can be more simple and can be realized at a lower cost.